Cat struggling to take breath

My cat struggles to take breath after 10-15 minutes of play. All the tests are done, and vet say nothing’s serious as of now but one of his values of heart valves slightly are at the edge., insignificantly low variance as per normal values. But let’s keep monitoring for few more days. Whenever we see him like this, we immediately stop playing so that he takes his time. Tried to take multiple opinions but no one’s able to figure out exact issue. Has anyone come across like this? Any similar experiences? Or is it normal?